Open potentiometer detection system

ABSTRACT

A system to detect whether a potentiometer is in an open circuit condition is disclosed. The system includes a potentiometer having a resistive element coupled between a voltage input and ground and an adjustable arm determining the resistance of the resistive element. A low pass filter is coupled to the adjustable arm. A controller has a first driver output coupled to the voltage input of the potentiometer and a second driver output coupled to the adjustable arm. The controller determines failure of the potentiometer by setting the first driver output coupled to the voltage input of the potentiometer to a high value. A first sample voltage from the adjustable arm is read and determined whether the first sample voltage is between a high threshold and a low threshold value. The first driver output coupled to the voltage input of the potentiometer is set to a low value. A voltage is applied to the adjustable arm via the second driver output. A second sample is read from the adjustable input and it is determined whether the second sample is below an arm threshold value.

TECHNICAL FIELD

The present disclosure relates generally to a potentiometer system andmore specifically to a detection system to determine the failure of apotentiometer.

BACKGROUND

Potentiometer applications include a wide variety of electronic devicesand uses within the devices. Potentiometers are commonly used to provideinput from sensors and controls, by achieving a defined relationshipbetween a mechanical position and a variable resistance. An electronicmeasurement system monitors the dependent resistance characteristic ofthe potentiometer to determine the mechanical position, and subsequentlyan output voltage related to the position of the device is determined.One example application of a potentiometer is an adjustment dial for auser interface to an electronic motor overload relay.

These electronic systems depend on defined performance characteristicsof the potentiometer. Potentiometer manufacturers define performancevariations over various criteria, such as application temperature or thevibration environment. However, eventually all potentiometers succumb tosome wear out and fail. When the potentiometer fails, the electronicsystem no longer receives the input that was controlled or monitoredthrough the potentiometer. This can correspond to a loss offunctionality in the device using the potentiometer because of the lossof electrical input from the monitored sensors or controls.

Today, multiple approaches exist to address the eventual wear out andresulting failure of the potentiometer. One approach uses analysis andtesting to demonstrate that the potentiometer will not fail over theservice life of the device. In this case, the failure is not activelydetected or mitigated in the application, however the robustness of thepotentiometer is deemed adequate to avoid a loss of functionality in thedevice. Another approach uses detection means to determine when thepotentiometer has failed, and to take some subsequent action. The actionmay include alerting the user, or entering a safe state such as shuttingdown the device.

Prior methods of detecting potentiometer failure exist. However theknown methods involve overhead and cost, including the need foradditional physical components, which are not suitable for allapplications. Known methods also include monitoring characteristics ofthe potentiometer, which may be important in some applications, but notimportant in others. An example application with constraints oncomponent cost that is impacted by specific failure modes of thepotentiometer is an electronic motor overload relay.

In such an electronic motor overload relay, a potentiometer may be usedas a voltage divider, where the divided voltage is determined by theposition of an adjustment dial for a user interface to set the motorfull load current parameter for the device. When the potentiometer isused as a voltage divider in this application, changes in someparameters of the potentiometer during the motor life such as change inthe resistance value over temperature, do not affect the performance ofthe potentiometer in the device. However, a failure in which anyconnection within the potentiometer becomes open circuit, either betweenmechanical interfaces of the potentiometer subcomponents or between thepotentiometer and the electronic board, can affect the performance ofthe device.

Thus, a need exists for a potentiometer failure detection system thatreliably detects the failure of a potentiometer. There is a further needfor a system that uses components for the reading of a potentiometer todetermine failures. There is also a need for a detection system that canidentify the specific source of the failure for the device in which itis permissible or preferred to continue operating in the presence ofcertain distinguishable potentiometer failures.

SUMMARY

One disclosed example is a system to detect whether a potentiometer isin an open circuit condition. The system includes the potentiometer,which has a resistive element coupled between a voltage input and groundand an adjustable arm determining the resistance of the resistiveelement. A controller has a first driver output coupled to the voltageinput of the potentiometer and a second driver output coupled to theadjustable arm. An analog to digital converter is also coupled to theadjustable arm to read the voltage of the potentiometer. The controllerruns a routine to determine failure of the potentiometer. The routinesets the first driver output coupled to the voltage input of thepotentiometer to a high value. A first sample voltage from theadjustable arm is read. It is determined whether the first samplevoltage is between a high threshold and a low threshold value. If thefirst sample is outside the high and low threshold values, apotentiometer failure may be detected. The first driver output coupledto the voltage input of the potentiometer is then set to a low value. Avoltage is then applied to the adjustable arm via the second driveroutput. A second voltage sample from the adjustable arm is read.Depending on whether the second sample is below an arm threshold value,a failure may be determined of the potentiometer.

Thus, the detection system allows the detection of failure modes of thepotentiometer, which are critical in particular applications,specifically an open circuit involving the potentiometer. The disclosedmethod does not add any cost in terms of additional components specificto the purpose of detecting the potentiometer failure. The use of amicrocontroller to detect the open circuit condition in a circuit usinga potentiometer is an additional advantage.

Additional aspects will be apparent to those of ordinary skill in theart in view of the detailed description of various embodiments, which ismade with reference to the drawings, a brief description of which isprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings.

FIG. 1A is a circuit diagram of a potentiometer failure detection systemand an associated controller;

FIG. 1B is the circuit diagram of the detection system in FIG. 1Ashowing potential points of failure in connections to the potentiometerresulting in potentiometer failure;

FIG. 2A is a timing diagram showing the signals used in monitoring thepotentiometer in FIG. 1A;

FIG. 2B is a timing diagram showing the signals used in monitoring thepotentiometer when no failure has occurred and the wiper arm is within anormal range of values;

FIG. 3 is a flow diagram of the control algorithm executed by themicrocontroller to detect failure of the potentiometer in FIG. 1A;

FIG. 4A is a timing diagram showing signals in the detection system inFIG. 1A when a first type of failure is detected;

FIG. 4B is a timing diagram showing the signals in the detection systemwhen the wiper arm of the potentiometer is set low and therefore nofailure has occurred;

FIG. 4C is a timing diagram showing the signals in the detection systemin FIG. 1A when a second type of failure is detected;

FIG. 4D is a timing diagram showing the signals in the detection systemwhen the wiper arm of the potentiometer is set high and therefore nofailure has occurred; and

FIG. 4E is a timing diagram showing the electrical signals in thedetection system in FIG. 1A when a third type of failure is detected.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. Rather, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

One disclosed example is a system to detect whether a potentiometer isin an open circuit condition. The system includes a potentiometer havinga resistive element coupled between a voltage input and ground and anadjustable arm. The position of the adjustable arm on the resistiveelement determines the voltage output at the adjustable arm. Acontroller has a first driver coupled to the voltage input of thepotentiometer and a second driver coupled to the adjustable arm. Thecontroller determines failure of the potentiometer by setting the firstdriver output coupled to the voltage input of the potentiometer to ahigh value. The controller then reads a first sample voltage from theadjustable arm and determines whether the first sample voltage isbetween a maximum threshold and a minimum threshold value. Thecontroller sets the first driver coupled to the voltage input of thepotentiometer to a low value. The controller applies a voltage to theadjustable arm via the second driver. The controller reads a secondsample from the adjustable input and determines whether the secondsample is above an arm threshold value.

Another example is a method of determining the failure of apotentiometer having an resistive element coupled between a voltageinput and ground and an adjustable arm determining the resistance of theresistive element. A first driver output coupled to the voltage input ofthe potentiometer is set to a high value. A first sample voltage is readfrom the adjustable arm. It is determined whether the first samplevoltage is between a high threshold and a low threshold value. The firstdriver output coupled to the voltage input of the potentiometer is setto a low value. A voltage is applied to the adjustable arm via a seconddriver output. A second sample is read from the adjustable input and itis determined whether the second sample is below an arm threshold value.

Another disclosed example is a method of determining the failure of apotentiometer having a resistive element coupled between a voltage inputand ground and an adjustable arm determining the resistance of theresistive element. The method uses a controller having a first driveroutput and a second driver output. The first driver output is coupled tothe voltage input of the potentiometer and the second driver output iscoupled to the adjustable arm. The first driver output coupled to thevoltage input of the potentiometer is set to a high value via acontroller. A first sample voltage is read from the adjustable arm aftera delay determined by the time constant of a low pass capacitor coupledto the adjustable arm. It is determined whether the first sample voltageis between a high threshold and a low threshold value. The first driveroutput coupled to the voltage input of the potentiometer is set to a lowvalue. A voltage is applied to the adjustable arm via the second driveroutput. A second sample is read from the adjustable input after a delaydetermined by the time constant of a low pass capacitor coupled to theadjustable arm. It is determined whether the second sample is below anarm threshold value via the controller.

FIG. 1A shows a potentiometer failure detection system 100 having amicrocontroller 102 coupled to a potentiometer 104 and a low passanti-aliasing filter 106. The potentiometer 104 includes an adjustablewiper arm 110, which is coupled to a resistive element 112. The low passfilter 106 is coupled to the adjustable wiper arm 110. The position ofthe wiper arm 110 determines the voltage between a voltage supply input114 and ground by dividing the resistive element 112. Thus, the voltageoutput at the wiper arm 110 is lowest when the wiper arm 110 is at theend of the resistive element 112 closest to ground. The voltage outputof the wiper arm 110 is the highest when the wiper arm 110 is movedclosest to the end of the resistive element 112 coupled to the voltagesupply input 114. As is commonly understood the wiper arm 110 isattached to a moveable physical object whose position is associated withthe position of the wiper arm 110 on the resistive element 112. Theoutput voltage of the potentiometer 104 at the wiper arm 110 isproportional to the position of the wiper arm 110 on the resistiveelement 112. The low pass filter 106 includes a resistor 120, which iscoupled to a capacitor 122 on the resistive element 112. The other endof the capacitor 122 is coupled to ground.

The microcontroller 102 includes an analog to digital converter (ADC)130, a first general purpose input/output (GPIO) driver 132, an analogto digital input 134, and a second GPIO driver 136. The microcontroller102 includes a voltage output pin 140 and an analog voltage input/outputpin 142. The microcontroller 102 uses the first general purposeinput/output (GPIO) driver 132 coupled to the voltage input 114 of thepotentiometer 104 via the voltage output pin 140 to apply a voltageacross the potentiometer 104. The voltage at the potentiometer wiper arm110 is filtered through the simple first order low pass anti-aliasingfilter 106 and input to the analog to digital converter (ADC) 130 viathe analog input/output pin 142. The analog to digital converter 130samples the analog input voltage signal from the wiper arm 110 andconverts it to a digital value for analysis by the microcontroller 102to detect failure of the potentiometer 104 because the potentiometer 104is in an open circuit condition as will be explained. In themicrocontroller 102, the analog input/output pin 142 is shared with thesecond GPIO driver 136 and may be toggled between an output function andan input function by a control signal to couple the second GPIO driver136 to the adjustable wiper arm 110.

The microcontroller 102 may be a microprocessor, a processor, anapplication specific integrated circuit (ASIC), a programmable logiccontroller (PLC), a programmable logic device (PLD), a fieldprogrammable logic device (FPLD), a field programmable gate array(FPGA), discrete logic, etc. or any other similar device. Themicrocontroller 102 may include a memory (not shown), which may includehardware, firmware, or tangible machine-readable storage media thatstore instructions and data for performing the operations describedherein. Machine-readable storage media includes any mechanism thatstores information and provides the information in a form readable by amachine. For example, machine-readable storage media includes read onlymemory (ROM), random access memory (RAM), magnetic disk storage media,optical storage media, flash memory, etc.

The sampling process may be performed according to the timing diagramshown in FIG. 2A. The timing diagram in FIG. 2A includes a GPIO outputsignal 202, a control voltage signal 204, and an ADC input voltagesignal 206. The GPIO output signal 202 is the voltage output from thevoltage output pin 140 of the microcontroller 102 coupled to the input114 of the potentiometer 104. The control voltage signal 204 toggles theinput/output pin 142 between the output function providing a voltagefrom the second GPIO driver 136 and the input function providing aninput signal from the wiper arm 110 to the ADC 130. The ADC inputvoltage signal 206 is the signal that is output from the wiper arm 110,which is read by the ADC 130.

To prepare to read the position of the wiper arm 110 on thepotentiometer 104, the GPIO output signal 202 from the first GPIO driver132 is set to a high value to provide voltage to the potentiometer 104at a first time period 210. The control voltage 204 is set high in orderto set the input/output pin 142 to accept an analog input signal fromthe wiper arm 110 at the first time period 210. After a delay periodsufficient for the time constant of the capacitor 122 of the low passanti-aliasing filter 106 used in the system 100, the ADC input voltagesignal 206 is sampled by the microcontroller 102 at a second time period212 and the position of the wiper arm 110 of the potentiometer 104 maybe determined from the value of the input signal 206 converted by theADC 130 to a digital value.

FIG. 1B shows the system 100 in FIG. 1A with potential areas for failureof the potentiometer 104. Like element numbers in FIG. 1A representtheir counterparts in FIG. 1B. Various potential disruption points suchas a first failure point 150, a second failure point 152, and a thirdfailure point 154 may each be the cause of potential breakdowns of thepotentiometer 104 resulting in loss of functionality of the device usingthe potentiometer 104. In certain applications such as a motor overloadrelay, this loss of functionality may result in subsequent damage to theload protected by the device. Each of the failure points 152, 154, and156 may result in an overload and failure of the potentiometer 104. Thefirst failure point 150 is at the connection between the potentiometerwiper arm 110 and the analog voltage input/output pin 142 coupled to theADC 130. The second failure point 152 is at the voltage input 114coupled to the resistive element 112. The third failure point 154 is atthe connection of the potentiometer 104 to ground.

In the disclosed example, the microcontroller 102 executes a routine toread the outputs from the potentiometer 104 and, based on thosereadings, determines whether the potentiometer 104 has failed. Theroutine also determines which of the three failure points 150, 152, or154 in FIG. 1B is the cause of the failure. FIG. 2B is a timing diagramthat shows the signals used in the process of determining potentiometerfailure. FIG. 2B includes the signals 202, 204, and 206 that are thevarious input and output signals explained in relation to FIG. 2A. Themeasurement process of the potentiometer 104 by the microcontroller 102proceeds in an identical sequence as that shown in FIG. 2A. The signals202, 204, and 206, and time periods 210 and 212 for this process aretherefore identical to their counterparts in FIG. 2A.

At a subsequent time period 220, the GPIO driver output signal 202 fromthe first GPIO driver 132 coupled to the voltage input 114 of thepotentiometer 104 is set to a low value, such that both ends of thepotentiometer 104 are at the same voltage. Also at the time period 220,the analog input/output pin 142 of the microcontroller 102 is configuredas a GPIO output by setting the control signal 204 low. This applies avoltage from the second GPIO driver 136 to the potentiometer wiper arm110, which is stored in the capacitor 122. After setting the GPIO driveroutput signal 202 low and the output signal of GPIO driver 136 high, adelay period occurs sufficient for the capacitor 122 to be charged. Thedelay period therefore is based on the time constant of the circuitformed by the low pass anti-aliasing filter 106 in parallel with theequivalent resistance of the potentiometer 104. At a subsequent timeperiod 222, the pin 142 of the microcontroller 102 is toggled foraccepting an ADC input by setting the control signal 204 high. Themicrocontroller 102 then waits for a maximum delay period that is lessthan the time constant of the circuit formed by the low passanti-aliasing filter 106 in parallel with the equivalent resistance ofthe potentiometer 104. After the delay, at a time period 224, the ADCinput voltage signal 206 is sampled. The microcontroller 102 detectswhether an open circuit potentiometer failure has occurred using thevoltage of the anti-aliasing capacitor 122 when sampled at the timeperiod 224.

Using the measurements taken by the process described above, themicrocontroller 102 is programmed to apply a decision algorithmdescribed in FIG. 3 to detect the potentiometer open circuit failuresshown in FIG. 1B.

The operation of the example decision algorithm to detect apotentiometer open circuit failure will now be described with referenceto FIGS. 1A-1B and 2B in conjunction with the flow diagram shown in FIG.3. The flow diagram in FIG. 3 is representative of examplemachine-readable instructions for implementing the processes describedabove to detect a potentiometer open circuit failure. In this example,the machine readable instructions comprise an algorithm for executionby: (a) a processor, (b) a controller, or (c) one or more other suitableprocessing device(s). The algorithm can be embodied in software storedon tangible media such as, for example, a flash memory, a CD-ROM, afloppy disk, a hard drive, a digital video (versatile) disk (DVD), orother memory devices, but persons of ordinary skill in the art willreadily appreciate that the entire algorithm and/or parts thereof couldalternatively be executed by a device other than a processor and/orembodied in firmware or dedicated hardware in a well-known manner (e.g.,it may be implemented by an application specific integrated circuit(ASIC), a programmable logic device (PLD), a field programmable logicdevice (FPLD), a field programmable gate array (FPGA), discrete logic,etc.). For example, any or all of the components of the microcontroller102 in FIG. 1A could be implemented by software, hardware, and/orfirmware. Also, some or all of the machine readable instructionsrepresented by the flowchart of FIG. 3 can be implemented manually.Further, although the example algorithm is described with reference tothe flowchart illustrated in FIG. 3, persons of ordinary skill in theart will readily appreciate that many other methods of implementing theexample machine readable instructions can alternatively be used. Forexample, the order of execution of the blocks can be changed, and/orsome of the blocks described can be changed, eliminated, or combined.

The decision algorithm in FIG. 3 applies three threshold values todetect open circuit failures of the potentiometer 104 in FIG. 1A. Aminimum threshold value and a maximum threshold value are set accordingto the optimal range of the potentiometer. The minimum and maximumthreshold values are used to determine whether the voltage of thepotentiometer is between the minimum and maximum threshold values thatrepresent the expected range of the potentiometer 104. An arm thresholdvalue is determined by the minimum value when the wiper arm 110 is movedto ground. The three threshold values are preferably stored in a staticinternal memory of the microcontroller 102 (not shown). Of course thevalues may be stored in dynamic memory, which allows a user to adjustthe threshold values.

The decision algorithm first reads the voltage output of the wiper arm110 of the potentiometer 104 from the ADC 130 according to the processdescribed in FIG. 2A above (300). The ADC value is determined from theinput voltage read by the ADC 130 from the wiper arm 110 (302).Following the default ADC measurement process, the ADC value from thewiper arm 110 of the potentiometer 104 is compared against minimum andmaximum threshold values (304). These values represent the nominal range(highest and lowest expected voltage) of the potentiometer 104 as usedin the system 100 in FIG. 1A. If the ADC value is within the nominalrange, no failure of the potentiometer 104 is detected and the algorithmends (306). If the ADC value is outside of the range established by theminimum and maximum threshold values, the potentiometer 104 may have anopen circuit failure and the algorithm provides an additional ADCmeasurement after a delay (308). After a second ADC value is determinedduring the process described above in FIG. 2B (310), the new value iscompared to an arm threshold value (312). If the second ADC value isabove the threshold value, the routine records a potentiometer opencircuit failure (314), otherwise if the second ADC value is below thearm threshold value, the algorithm detects that while the original ADCmeasurement value was outside the nominal range of values there is noopen circuit failure (316).

Returning to FIG. 1B, the cause of failure of the potentiometer 104 maybe determined based on analysis of the signals input to themicrocontroller 102 during the sequence described above in FIG. 2B. Auser may therefore determine whether the disconnection causing thefailure is at one of the failure points 150, 152, or 154 in FIG. 1B.FIG. 4A is a timing diagram showing the signals 202, 204, and 206 inFIG. 2A that result when a failure occurs such as at the failure point150 in FIG. 1B. The different time periods of the process in FIG. 2B areshown with like element numbers in FIG. 4A.

As shown in FIG. 4A, a first ADC measurement value 402 from theinput/output pin 142 will be at the minimum value of the ADC inputbecause there is no current through the potentiometer 104 to the filtercapacitor 122 of the low pass filter 106 to create a voltage. Bycomparison of the ADC value 402 with the maximum and minimum thresholdvalues that are stored by the microcontroller 102, the ADC value 402 isoutside the parameters (lower than the low threshold value), whichcauses additional measurements to be made as shown at the time period222 in FIG. 4A. Thus, the GPIO output 202 is driven low to create a lowvoltage to the potentiometer 104. The control signal 204 to configurethe analog input/output pin 142 is set to low to configure theinput/output pin 142 as a GPIO output. The voltage is raised from thesecond GPIO driver 136 through the pin 142 and current flows through thecapacitor 122 creating a stored voltage. After a delay period of thetime constant of the low pass filter 106, the control voltage signal 204is set low to toggle the input/output pin 142 to accept an analog signalto the ADC input 134 at the time period 222. The signal from the wiperarm 110 representing the stored voltage on the capacitor 122 is sampledby the ADC 130 at the time period 224 and the second thresholdcomparison is performed by the microcontroller 102. As shown in FIG. 4A,an ADC input voltage level 404 from the second sample is at a relativelyhigh level. The comparison of the input voltage level 404 is made withthe arm threshold value by the microcontroller 102. Since the secondinput voltage level 404 exceeds the arm threshold value in this example,the microcontroller 102 determines that a failure has occurredpotentially at the failure point 150 in FIG. 1B.

FIG. 4B is a timing diagram of the signals 202, 204, and 206 in FIG. 2Athat are used to determine whether a failure has occurred from adisconnection at the failure point 152 in FIG. 1B. As previouslyexplained, the GPIO output voltage 202 is set high to apply voltage tothe potentiometer 104. In this case, an ADC input voltage 412 from thewiper arm 110 is at a minimum value below the minimum threshold value.This may be because no current is flowing through the potentiometer 104due to the break from the failure point 152 at the voltage input 114 inFIG. 1B. This failure may result in a low voltage value at theinput/output pin 142, regardless of the actual position of the wiper arm110 based on the minimum value measurement of the ADC input as shown bythe signal value 412 at the time period 212 in FIG. 4B. However, the lowinput voltage may also be because the wiper arm 110 is positioned at theend of the resistive element 112 closest to ground.

During the second ADC measurement at the subsequent time period 220, thepin controlling the analog input/output pin 142 is raised to a highvalue and configures the pin 142 as a GPIO output coupled to the secondGPIO driver 136. Thus, the current flow into the capacitor 122 isdetermined by the position of the wiper arm 110 on the resistive element112. In this case, the circuit formed by the low pass filter 106 and thepotentiometer 104 forms a voltage divider.

If the wiper arm 110 is positioned at the end of the resistive element112 coupled to ground, the voltage across the capacitor 122 when currentis applied from the voltage from the second GPIO driver 136 will be low.Thus, a second ADC sample value 414 is not above the threshold value asshown in FIG. 4B and therefore the microcontroller 102 may determinethat no failure has occurred since the wiper arm 110 is properly at theend of the resistive element 112 causing the initial ADC voltage value412 to be below the minimum threshold value.

FIG. 4C shows the signals 202, 204, and 206 when an actual failure isdetected at the failure point 152 in FIG. 1B. As in FIG. 4B, a firstvoltage sample 422 is below the low threshold value indicating apotential failure. The microcontroller 102 will proceed to take a secondADC value 424 at the time period 224. The second ADC sample value 424 iscompared with the second threshold value. If the second ADC sample value424 is above the arm threshold value as shown in FIG. 4C, themicrocontroller 102 determines that failure has occurred at the failurepoint 152 in FIG. 1B. If the potentiometer 104 is not coupled to thefirst GPIO driver 132, the capacitor 122 will store voltage andtherefore return a high value with the second sample at the time point224. As explained above, if both ADC samples at time periods 212 and 224are low, the microcontroller 102 determines that the wiper arm 110 isset at the end of the resistive element 112 coupled to ground andtherefore no failure has occurred.

FIG. 4D is a timing diagram showing the signals 202, 204, and 206 inFIG. 2B reflecting the detection of a possible disconnection at failurepoint 154 in FIG. 1B. After the time period 212, the voltage from thewiper arm 110 is measured by the ADC 130 via the input/output pin 142and compared with the minimum and maximum threshold values. As shown inFIG. 4D, an ADC input voltage value 432 is greater than the maximumthreshold value and therefore a potential failure condition may exist atthe failure point 154 in FIG. 1B. However, the input voltage value 432being above the threshold range may be a result of the wiper arm 110being positioned at the end of the resistive element 112 coupled to thevoltage input 114. The process continues with the control voltage 204being set low to cause the pin 142 to output voltage from the secondGPIO output driver 136 to the wiper arm 110 to charge the capacitor 122.The control voltage signal 204 is then set low and a sample is taken bythe ADC 130 of the input voltage signal 206 at the time point 224. Asshown in FIG. 4D, an ADC input signal value 434 is low, which indicatesthat the wiper arm 110 is in a position near the voltage input 114 onthe resistive element 112. Thus, the routine on the microcontroller 102determines that no failure has occurred.

FIG. 4E is a timing diagram showing the signals 202, 204, and 206 inFIG. 2B reflecting the detection of a disconnection at the failure point154 in FIG. 1B. In FIG. 4E, the first sample of the ADC input signal 206is a high signal while a second sample value 444 of the ADC input signal206 after the time period 224 is a high value that exceeds the armthreshold value. Since the second sample value 444 of the ADC inputsignal 206 is a high value, the microcontroller 102 determines that afailure of the potentiometer 104 has occurred at the failure point 154.

While the present invention has been described with reference to one ormore particular embodiments, those skilled in the art will recognizethat many changes can be made thereto without departing from the spiritand scope of the present invention. Each of these embodiments andobvious variations thereof is contemplated as falling within the spiritand scope of the claimed invention, which is set forth in the followingclaims.

What is claimed is:
 1. A system to detect whether a potentiometer is inan open circuit condition, comprising: a potentiometer having aresistive element coupled between a voltage input and ground and anadjustable arm, the position of the adjustable arm on the resistiveelement determining the voltage output at the adjustable arm; and acontroller having a first driver coupled to the voltage input of thepotentiometer and a second driver coupled to the adjustable arm, whereinthe controller determines failure of the potentiometer by: setting thefirst driver output coupled to the voltage input of the potentiometer toa high value; reading a first sample voltage from the adjustable arm anddetermining whether the first sample voltage is between a maximumthreshold and a minimum threshold value; setting the first drivercoupled to the voltage input of the potentiometer to a low value;applying a voltage to the adjustable arm via the second driver; andreading a second sample from the adjustable input and determiningwhether the second sample is above an arm threshold value.
 2. The systemof claim 1, wherein the controller includes a switchable input coupledto the adjustable arm, the switchable input being switchable between ananalog to digital converter and the second driver output, the controllercontrolling the switchable input between coupling the analog to digitalconverter to the adjustable arm to read the first and second samplevoltages and coupling the second driver to apply the voltage to theadjustable arm.
 3. The system of claim 1, wherein the controller is amicrocontroller.
 4. The system of claim 1, wherein a failure of adisconnection of the adjustable arm is determined when the first samplevoltage is below the minimum threshold value and the second samplevoltage is above the arm threshold value.
 5. The system of claim 1,wherein a failure of a disconnection of the resistive element to thepower input is determined when the first sample voltage is below theminimum threshold value and the second sample is above the arm thresholdvalue.
 6. The system of claim 1, wherein a failure of a disconnection ofresistive element from ground is determined when the first sample isabove the maximum threshold value and the second sample is above the armthreshold value.
 7. The system of claim 1, wherein the threshold valuesare associated with the operating parameters of the potentiometer. 8.The system of claim 1, wherein the threshold values are stored in staticmemory in the controller.
 9. The system of claim 1, further comprising alow pass filter including a capacitor coupled to the adjustable arm,wherein the reading the first and second samples is delayed for a timeperiod associated with the time constant of the capacitor of the lowpass filter.
 10. A method of determining the failure of a potentiometerhaving a resistive element coupled between a voltage input and groundand an adjustable arm determining the resistance of the resistiveelement, the method comprising: setting a first driver output coupled tothe voltage input of the potentiometer to a high value; reading a firstsample voltage from the adjustable arm and determining whether the firstsample voltage is between a high threshold and a low threshold value;setting the first driver output coupled to the voltage input of thepotentiometer to a low value; applying a voltage to the adjustable armvia a second driver output; and reading a second sample from theadjustable input and determining whether the second sample is below anarm threshold value.
 11. The method of claim 10, wherein a failure of adisconnection of the adjustable arm is determined when the first samplevoltage is below the minimum threshold value and the second samplevoltage is above the arm threshold value.
 12. The method of claim 10,wherein a failure of a disconnection of the resistive element to thepower input is determined when the first sample voltage is below theminimum threshold value and the second sample is above the arm thresholdvalue.
 13. The method of claim 10, wherein a failure of a disconnectionof resistive element from ground is determined when the first sample isabove the maximum threshold value and the second sample is above the armthreshold value.
 14. The method of claim 10, wherein the thresholdvalues are associated with the operating parameters of thepotentiometer.
 15. The method of claim 10, wherein the threshold valuesare stored in static memory.
 16. The method of claim 10, wherein a lowpass filter including a capacitor is coupled to the adjustable arm, andwherein the reading the first and second samples is delayed for a timeperiod associated with the time constant of the capacitor of the lowpass filter.
 17. A method of determining the failure of a potentiometerhaving a resistive element coupled between a voltage input and groundand an adjustable arm determining the resistance of the resistiveelement using a controller having a first driver output and a seconddriver output, the method comprising: coupling the first driver outputto the voltage input of the potentiometer and the second driver outputto the adjustable arm; setting the first driver output coupled to thevoltage input of the potentiometer to a high value; reading a firstsample voltage from the adjustable arm after a delay determined by thetime constant of a low pass capacitor coupled to the adjustable arm;determining whether the first sample voltage is between a high thresholdand a low threshold value; setting the first driver output coupled tothe voltage input of the potentiometer to a low value; applying avoltage to the adjustable arm via the second driver output; reading asecond sample from the adjustable input after a delay determined by thetime constant of a low pass capacitor coupled to the adjustable arm; anddetermining whether the second sample is below an arm threshold valuevia the controller.
 18. The method of claim 17, wherein a failure of adisconnection of the resistive element to the power input is determinedwhen the first sample voltage is below the minimum threshold value andthe second sample is above the arm threshold value.
 19. The method ofclaim 17, wherein a failure of a disconnection of resistive element fromground is determined when the first sample is above the maximumthreshold value and the second sample is above the arm threshold value.